This question has absorbed more than just science fiction writers. Scientists have been plucking away at the question for years, using direct and indirect measurements the conditions on Mars to figure out what life there would be life—if it exists, or every did.

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In 2009, for example, the Phoenix Lander directly confirmed there to be water ice on the Martian surface, Washington State University researcher Dirk Schulze-Makuch told Popular Mechanics then, "I'd be puzzled if Mars were completely sterile. Once life got entrenched, there would be some way for life to sustain itself in ecological niches.”

Living Underground

NASA/Viking 1

Thanks to the new study, one such niche has now been found in the form an underground aquifer filled with briny water. The water is at least twice as salty as seawater. In some ways, that's good for life—saltwater doesn’t freeze as easily as freshwater, which is important because the water under Mars would be between negative 10 and negative 30 degrees Celsius. But there's also bad news for life. The Mars water would be full of perchlorate salt, which chemically toxic to most Earth lifeforms.

Most, but not all. Bacteria called Dechloromonas aromatica reduce perchlorate, using enzymes to break it down into chloride and oxygen, which it breathes. These little guys actually move toward concentrations of perchlorate. They are called chemotropic organisms, because the lifeform's growth is influenced by some sort of external chemical stimuli (a phototroph, meanwhile, lives on the surface and derives chemical energy from sunlight).

Perhaps deep under the ground of Mars, organisms use chemicals to survive just like they do on the seafloor of Earth. Samuel Kounaves, Professor of Chemistry at Tufts University and Lead Scientist for the Wet Chemistry Lab on NASA’s Phoenix Mars Lander, recently told AstrobiologyMagazine that “there could be some chemotropic bacteria that uses perchlorate as an energy source – there could be a whole ecosystem that even had liquid water available to it.”

Methane Mysteries

NASA JPL/Caltech

In December 2014, NASA’s Curiosity rover detected a spike in methane ten times the normal level. This led to some conclusions that things were happening deep underground—lifeforms were creating that gas.

Curiosity has performed some interplanetary geology since then. NASA now says the consensus is that “water-rock chemistry might have generated the methane, but scientists cannot rule out the possibility of biological origins.” This methane would be seeping from underground aquifers, like the one spotted from Martian orbit. Methane-based life could also be possible, replacing oxygen as the critical ingredient, Many who study Titan see this possibility.

How far could this hypothetical underground Martian ecosystem grow? Chemotrophs tend to grow fast. Take Hydrogenovibrio crunogenus, a bacteria that thrives by converting sulfur from geothermal vents on the seafloor into oxygen. Those bacteria can double their numbers every hour, if enough sulfur is present. We may want to be careful about drilling and returning samples to Earth.

Complex Life?

A black smoker at the bottom of the Atlantic Ocean.

NOAA

Could the Martian environment support more complex organisms? In the deep-sea vents on Earth, larger animals like shrimp get a helping hand from the Earth’s heat seeping through the vents and organic material that drops from the surface world. Some crabs and shrimp live elsewhere, where a layer of bacteria form on their bodies that they will later eat while at the vent.

It’s hard to tell what could happen beneath Mars over the millennia. The amount of chemical energy available may limit the size of creatures under Mars, but it’s best not to rule anything out. Organisms in cold environments tend to survive by forming symbiotic relationships with other organisms, so the interplay between Martian bacteria could be hard to realize but sublime in its meaning.

The toughest Earth creature could be the water bear, or tardigrades, which are .04 inch long invertebrate animals that live in mosses, soil, or lichens on earth. They can survive temperatures from negative 253 degrees C to 151 C. So the cold wouldn’t kill them. But waterbears, like most advanced organisms, also rely on some oxygen to survive.

When it comes to life on Mars, there are certainly possibilities. Colonies of bacteria could produce oxygen from perchlorate, the very same chemical that keeps the water from freezing. If these bacteria can produce oxygen, maybe something larger and hardy could survive. Maybe the methane provides an alternate chemical that can help foster life.

We won’t know until we drill.

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